Here's a list of the Telltale games that are available again:
The rights to Batman: The Telltale Series, Batman: The Enemy Within, Hector: Badge of Carnage, Nelson Tethers: Puzzle Agent, Puzzle Agent 2, and The Wolf Among Us, were acquired by LCG Entertainment. It also acquired the publishing rights to RGX Showdown. These rights were secured with their formation of Telltale 2.0 on August 28, 2019. They later acquired the rights to Tales of Monkey Island on June 26, 2020, and Wallace and Gromit's Grand Adventures on November 7, 2020.
The rights to Telltale's The Walking Dead games were acquired by the creator of The Walking Dead, Skybound. They are now published by Skybound Games, a subsidiary that contracted members of the Telltale staff to finish The Walking Dead: The Final Season from the Telltale 1.0 offices in 2019. They also released a remastered version of all of Telltale's The Walking Dead games titled The Walking Dead: The Telltale Definitive Series.
As mentioned above, the rights to Tales from the Borderlands were acquired by 2K subsidiary Gearbox, the company that holds the rights to the Borderlands franchise.
The rights to the Sam & Max games were acquired by Skunkape Games, a company formed by former Telltale 1.0 staff. This is the studio that's currently remastering the games.
These games are currently available again to purchase, plus Sam & Max Save the World is available to pre-order in its remastered form. Sam & Max Beyond Time and Space Remastered and Sam & Max: The Devil's Playhouse Remastered will be available once Skunkape remasters those as well.
The rest are not available for purchase, although all are available to re-download if you purchased them from GOG.com, Steam, the Telltale Store, PlayStation Store, Microsoft Store, Nintendo e-Shop, etc. These remaining games presumably remain in rights limbo. I'll keep you updated as, hopefully, more legacy Telltale games become available to purchase again.
Update February 22, 2023: It's been two years to the day since this list was posted here. Since then, Sam & Max Save the World Remastered and Sam & Max Beyond Time and Space Remastered were released. Sam & Max: The Devil's Playhouse Remastered will be released in late 2023. The original Sam & Max games are available as a bonus with the remasters of the Windows versions.
Plus, Tales from the Borderlands received a sequel and there is now a Sam & Max virtual reality game. Additionally, if all goes as planned, Telltale 2.0 will release its first games in late 2023, including the long-awaited sequel to The Wolf Among Us.